“Shading” that occurs in an image pickup apparatus such as a digital still camera is a phenomenon in which brightness and/or color difference (unevenness) between a central portion and a peripheral portion in a picked up picture occurs due to peripheral light-reduction in a camera optics system, distribution of the properties of a solid-state image pickup device, and the like.
An image-correction method for correcting such shading is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2000-69371. In particular, the image-correction method in an image pickup apparatus disclosed in FIG. 1 in Embodiment 1 is adapted to implement shading correction, by calculating a shading-correction coefficient approximated by a two-order (an N-order) curved-surface function of the coordinates of pixels, and by multiplying an output signal from a CCD (Charge Coupled Device) by the shading-correction coefficient.
However, because, in such a conventional image-correction method, a shading-correction coefficient approximated by a two-order (an N-order) curved-surface function of the coordinates of pixels is calculated, and because shading is a phenomenon that is caused by a plurality of factors such as lens properties and sensitivity properties of a solid-state image pickup device, there is a problem in that the approximation by an N-order function may not implement the shading correction with sufficient accuracy.
In addition, an image-correction method for correcting shading is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2002-216136. In particular, the image-correction method in an image pickup apparatus disclosed in FIG. 12 and Page 7 is adapted to implement shading correction, by calculating a distance between coordinates of a pixel and coordinates of a light-axis center and then taking out from a look-up table a shading-correction coefficient corresponding to the distance, and by multiplying an output signal form a CCD by the shading-correction coefficient.
However, in such a conventional image-correction method, in order to enhance the accuracy of the shading correction, a great deal of data should be held in a look-up table. Moreover, in the case where, due to combination with an optical zooming function and the like, the properties of shading dynamically varies during picking up images, changing of the data is required; therefore, there is a problem in that, more storage areas are required in order to hold alteration data, and processing time necessary for the alteration is prolonged, thereby causing deterioration in performance such as decrease in a frame rate for picking up an image.
As described above, in conventional image-correction methods, there has been a problem in that shading correction is not implemented with sufficient accuracy. In addition, in another conventional image-correction methods, there has been a problem in that, because a great deal of data is included in a look-up table, many storage areas are required in order to hold the data, thereby causing deterioration in performance.
It is an object of the present invention to solve the foregoing problems that the conventional image-correction methods possess.